This invention relates to a transmission failure diagnosis apparatus in a multiplex transmission system which diagnoses failures occurring in a system, for example, line disconnection of a transmission path or failures of transmission functions of nodes and so forth, in a multiplex transmission system which sends and receives data mutually by a time division multiplex communication via a common transmission path among a plurality of nodes. This multiplex transmission system has numerous applications, for example, in a large-scale communication system, LAN in an intelligent building, a bus system in the home, etc.
In recent years, particularly in the field of automobiles, a multiplex transmission system which sends and receives a signal between nodes positioned at various points of a vehicle by using a common multiplex transmission path has been introduced for the purpose of preventing the number of signal lines needed for electronic control from becoming large. Such a multiplex transmission system is disclosed, for example, in Japanese Patent Publication for Opposition (Kokoku) No. 61-32173.
In this kind of multiplex transmission system for vehicles, high reliability for the system is required. For example, to improve reliability (e.g., resistance to noise) against failures in a multiplex transmission path, a twisted pair line made of a pair of signal lines is used as multiplex transmission path, and transmission reception circuits at respective nodes are of a differential input/output type. Therefore, even if one of the signal lines of the twisted pair line is disconnected, signal transmission can be effected.
In the above-mentioned multiplex transmission system in which a multiplex transmission path is made highly reliable by the use of twisted pair lines, even if one of the signal lines of a twisted pair line is disconnected, the transmission and reception of a signal can be normally effected by using the other signal line. If a failure occurs in the transmission driver circuit of a node, similar phenomena will occur.
Such failures cause transmission data errors and result in an increase in the number of retransmissions. One of the great problems due to such failures is that they result in "data garble," or an increase in the number of retransmissions, so the user does not notice these failures in the multiplex transmission path.
As a result of this, in spite of the fact that high reliability of a multiplex transmission path is achieved by twisted pair lines, the multiplex transmission system remains usable through the agency of a multiplex transmission path which has become low in reliability, after a failure occurs in one of the signal lines of a twisted pair line without being noticed. This fact is a great problem in a system which requires high reliability.